The present invention relates generally to the fabrication of a semiconductor device and more particularly to a system and a method for characterizing lithography test structures formed using a stepper system and using that information to calibrate focus and exposure settings of the stepper system.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution lithographic processes. In general, lithography involves the transfer of a pattern or image from one medium to another, as from a mask to a wafer. In particular, a mask can be used to shield one area of the wafer while exposing another in a lithography stepper system. For example, a photoresist is applied to a wafer which is then aligned to a mask. Then, an illumination source such as ultraviolet or x-ray radiation can be projected through the mask, thereby exposing the photoresist with the mask pattern. The wafer is then indexed, or xe2x80x9csteppedxe2x80x9d via a wafer stage system, and the image transfer is repeated at another location on the wafer. The wafer is then developed to remove the exposed photoresist and baked to harden the remaining photoresist pattern. Areas of the wafer not covered by the hardened photoresist are then etched away or otherwise processed, and the wafer is inspected to ensure the image transfer from the mask to the top layer is correct. This process is repeated several times until all of the active devices and features have been formed.
During the image transfer process of fabricating a semiconductor, many variables can affect the critical dimensions of fabricated features on the wafer. Specifications must be held within very close tolerances to meet device performance requirements, thus requiring precise adjustment of system variables. Calibration of variables within a lithography stepper system based upon an ideal control standard is typically required on a periodic basis to achieve a uniform process. For example, an in-house control specimen, or xe2x80x9cgolden standardxe2x80x9d, can act as a base precision standard, from which process parameters can be adjusted to account for variations in the system. The calibration using the xe2x80x9cgolden standardxe2x80x9d is typically performed manually, and can be subjective and time consuming. In an exemplary manual calibration procedure, test structures are exposed and developed on a test wafer with multiple focus and/or exposure conditions. The test wafer is then inspected via optical microscope and compared to an ideal standard. Upon visual analysis, if a perceived disparity occurs between the test wafer and the ideal standard, a correction is made to the focus and/or exposure settings and the procedure is repeated. Thus, due to the manual calibration inefficiencies and subjectivity, an efficient and objective means for calibration of the lithography stepper system utilizing an ideal, or xe2x80x9cgolden standardxe2x80x9d is desired to increase precision in the image transfer process.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention generally relates to the fabrication of a semiconductor device and more particularly to a system and a method for characterizing lithography test structures formed using a stepper system. The characterization information is used in conjunction with information from a predetermined ideal standard to calibrate focus and exposure settings of the stepper system.
The calibration of focus and exposure settings in a lithography stepper system can vary the structural profile of features formed on a wafer and thus affect an overall performance of a semiconductor device. Defective features may result in reduced performance of the semiconductor device. Thus, there is a need to accurately calibrate the focus and exposure settings of a stepper system before the actual production runs of wafers begins. According to one aspect of the invention, a calibration is accomplished by obtaining information from generated calibration test structures (e.g., gratings) from the stepper system for a variety of focus and exposure conditions and comparing the information to an ideal sample or xe2x80x9cgolden standardxe2x80x9d in order to ascertain an optimal focus and exposure condition and thereby increase the quality and precision of a fabrication process prior to production runs.
The present invention is directed to a system and a method for calibrating focus and exposure settings in a lithography stepper system. A stepper system is employed to print a plurality of test structures (e.g., a grating), at a variety of focus and exposure conditions. The generated plurality of test structures are further developed, wherein each has feature characteristics which are a function of their associated focus and exposure conditions. Each of the test structures is then measured and characterized, for example, using scatterometry, to generate characterization data associated therewith. The characterization data is then compared to data representing ideal feature characteristics (e.g., a xe2x80x9cgolden standardxe2x80x9d data set) via, for example, a correlation to determine an optimal focus and exposure condition according to a predetermined criteria. Calibrating the focus and exposure conditions to an ideal xe2x80x9cgolden standardxe2x80x9d at the beginning of a production run facilitates achieving desired critical dimensions at production startup.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.